The invention relates to a progressive power spectacle lens with a true short progression.
Because smaller spectacle frames have become fashionable, many manufacturers are at present induced to offer progressive power spectacle lenses having a shortened progressive length.
However, there is no generally valid definition of the length of a progressive zone. An indication of a vertical distance between stamp-marked points, for example between a distance and near reference point or between a centration point and a near reference point, is not sufficient to characterize the progressive length of a progressive power lens because the stamped marking need not necessarily correlate with the xe2x80x9cas wornxe2x80x9d properties, and for many lenses does not correlate.
A lens with a short progressive length may be expected to have approximately the power needed for correcting a defect of vision at the centration point or at the distance reference point, and to attain the power needed for near viewing well within a lens rim of a frame. The frame should not pass across either the distance portion or the near portion; rather than this, both portions should be located within the frame.
Extensive wearing tests which were performed by the inventors have shown that spectacle wearers find it agreeable not having to lower the gaze excessively, for example when reading a book. Less raising of the head, for example when working with a computer, is generally felt to be more agreeable ergonomically.
This calls for defining the progressive length as follows:
The length of a progressive zone is the difference between the vertical coordinates of two points, the upper point of which is that point on or directly adjacent to the principal visual axis at which the power for approximately correcting a defect of distant vision is present (e.g. the distance centration point), and the lower point of which is that point on or directly adjacent to the principal visual axis at which, when the glance is lowered, the near power stipulated by a prescription is attained for the first time.
This definition provides a measure of the progressive length which is independent from arbitrary stamped markings and takes account of the wearing situation in which a spectacles wearer uses the spectacles.
On the basis of this definition, a product comparison of spectacle lenses on the market shows that the indications of generally 12-14 mm, as given by most manufacturers for the progressive lengths of their products, do not withstand a realistic examination. The only products having true progressive lengths of 12 mm or 13 mm are xe2x80x9cKodak Concisexe2x80x9d brand and xe2x80x9cShamir Piccoloxe2x80x9d brand, respectively.
The designing of a progressive power lens having a short progressive zone is beset by some problems. One of the main problems is the increase of the surface astigmatism in a lateral direction on the right-hand side and the left-hand side of the principal (visual) axis, which is substantially described by Minkwitz"" Law. As the increase of the power value along the vertical direction becomes steeper, the increase of the surface astigmatism in the lateral direction becomes greater. Thus, the following tendency will apply:
As the progressive zone becomes shorter, the attained addition power being the same, it also becomes narrower.
This large gradient of the central astigmatism is also accompanied by an increase of maximum astigmatism at the periphery, which reduces the optical quality at the periphery and also affects the optical properties in the region of the principal meridian.
It is an object of the present invention to provide a spectacle lens having a (true) short progression zone in the sense of the above definition, and having, despite this requirement, advantageous imaging properties and an acceptable dynamic performance.
The achievement of this object in accordance with the invention has been obtained by providing a progressive length less than or equal to 12 mm and a width of the distance in a horizontal meridian y=+7 mm in dependence on power and an addition power larger than certain values as will be set forth below.
According to the invention, the distance portion advantageously keeps the horizontal change of power in the wearing position as small as possible. With practically all known progressive power lenses the power in the distance region increases in the outward direction from the principal meridian. This increase tends to be large particularly with conventional products having a shortened progressive length. This troublesome increase can be significantly reduced with a suitable surface design.
With progressive power lenses the power at the distance reference point is attained, for example, in the vertex measurement position or in the wearing position. In the first case the lens then has in the wearing position a power which as a rule differs only slightly from that prescribed. When shifting the gaze horizontally, a spectacle wearer experiences too strong a positive power along the outside direction; so-called xe2x80x9cfoggingxe2x80x9d occurs which is noticed and experienced by the spectacles wearer as being disturbing.
The widths of the distance portion depend on the power and the addition power of the progressive spectacle lens. As the addition power becomes greater, as a rule the widths of the distance portions become smaller.
Table 1 shows the widths of the distance portion of a progressive power lens according to the invention. Progressive power lenses (having a true short progression length) according to the state of the art generally have smaller widths of the distance portions. The coordinate system has been so chosen that in the wearing position the y axis extends vertically and the x axis extends horizontally.
The addition power and the spherical power in the distance portion (Sph) are given in diopters.
Along a horizontal meridian having the coordinate y=const.=7 mm, located 3 mm above the centration point and 1 mm below the distance reference point, the power of a progressive power lens according to the present invention differs substantially from the value prescribed for distance only very far on the outside, i.e. at large values of the horizontal coordinates.
The width of the distance portion along a given horizontal meridian is defined as the distance between the two points on the right-hand side and left-hand side of the principal meridian, at which the mean power exceeds the value of the spherical equivalent (sph+{fraction (1/2 )} cyl) of the prescription value by 0.5 dpt.
According to the invention it is also possible to significantly reduce the dynamic distortion with a suitable design of the surface.
The magnification (in percent) is defined as being the quotient of the retinal image size with a spectacle lens and the retinal image size without a spectacle lens. From this the following generally known formula may be derived:   Γ  =            tan      ⁢              xe2x80x83            ⁢              w        xe2x80x2                    tan      ⁢              xe2x80x83            ⁢      w      
in which wxe2x80x2 is the angle of sight with the spectacle lens and w is the angle of sight without the spectacle lens.
For small values of power, the magnification along horizontal meridians in the near portion follows a characteristic W shaped course. On or close to the principal meridian the magnification has a local maximum. From this maximum it at first decreases towards the outside, passes through a minimum, and then again increases. For this, the lens need not be symmetrical to the principal line; in particular, the temporal minimum may have a lower value than the nasal minimum.
A progressive power lens according to the invention is distinct in that the magnification changes only little along horizontal meridian.
In accordance with the invention, the maximum rise of the magnification, i.e. the difference between the magnification at the maximum (on or close to the principal meridian) and at the lower of the two minima, serves as a further feature. This is illustrated in Table 2: